This invention relates to thermionic converters, and especially to thermionic converters utilized in nuclear reactors.
One type of nuclear reactor includes an array of thermionic converter units that contain quantities of nuclear fuel. The nuclear fuel heats the emitters of the units while streams of fluid cool the collectors of the units, to create a temperature difference that results in the generation of current. One type of reactor which is intended for use on extra-terrestrial vehicles utilizes an array of emitter rods constructed of tungsten to permit operation at high temperatures. Passages in the tungsten rods hold pellets of nuclear fuel that heat the rods. Collectors are positioned close to certain emitter surfaces to collect electrons leaving the emitter and thereby create electricity.
One problem encountered, particularly in the case of small reactors, is the efficient utilization of the nuclear fuel. The fuel must be closely packed in order to cause the reactor to go critical with a minimum of fuel. However, space must be provided between the nuclear pellets to hold emitters that are to be heated by the fuel and to provide space for the cooled collectors that are positioned close to the emitters. Generally, many converter units are utilized which are electrically connected in series and which must be spaced from one another to prevent electrical shorting. An arrangement of emitters, collectors, and nuclear fuel which permitted close packing of the fuel while providing for sufficient heating of the emitters by the fuel and efficient cooling of the collectors, all in a structurally sound arrangement, would permit the construction of compact and reliable nuclear reactors.
Considerable attention is given to the construction of the emitters used in the nuclear reactors, inasmuch as these elements generally must withstand the highest temperatures and provide structural strength. Tungsten is often utilized, inasmuch as it has a relatively high vacuum work function and can withstand very high temperatures. The work function of tungsten varies somewhat, in a range such as 4.2 to 5.2 volts, with the work function generally being at the lower end of the range for the relatively pure tungsten that has been often utilized in emitters. The work function has been raised by the vapor deposition of tungsten on the emitter surfaces of the tungsten rods, using first a vapor deposition from tungsten hexafloride to obtain a high strength base layer of tungsten and then using a vapor deposition from tungsten hexachloride to obtain a tungsten layer with a 110 crystalographic orientation which produces a high vacuum work function. However, the vapor deposition process is expensive. The efficiency of operation of the emitter has also been found to improve by the addition of small amounts of oxygen to the emitter surface. This also is an expensive process. A tungsten emitter material which could operate efficiently without requiring vapor depositions of tungsten or the addition of oxygen to the emitter surfaces could be constructed at lower cost.